Continental Reactivation and Reworking: an Introduction

Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 Continental reactivation and reworking: an introduction R. E. HOLDSWORTH l, M. HAND 2, J. A. MILLER 3 & I. S. BUICK 4 1 Reactivation Research Group, Department of Geological Sciences, Durham University, Durham DH1 3LE, UK 2 Department of Geology and Geophysics, Adelaide University, Adelaide, SA 5005, Australia 3 Department of Geological Sciences, The University of Cape Town, Rondebosch 7700, Republic of South Africa 4 Department of Earth Sciences, La Trobe University, Bundoora, VIC 3083, Australia In contrast to oceanic lithosphere, the continents are linked to the on-going Alpine collision (e.g. are manifestly composed of the products of tec- Ramandi 1998). In the ancient geological record, tonic processes whose cumulative duration spans two of the best examples are the mid-Palaeozoic much of the Earths history. Most continents Alice Springs Orogeny and the Neoproterozoic contain Archaean nuclei that are enclosed by to Palaeozoic Petermann Orogeny in central Proterozoic and Phanerozoic tectonic domains. Australia (e.g. Sandiford & Hand 1998; Hand & The evolution of post-Archaean continental Sandiford 1999). In recognition of the impor- volumes has included additions of new continen- tance of intraplate orogeny as an expression tal material, but it has also involved repeated of continental rejuvenation, a large amount of modification of parts of the existing continental work has focused on the mechanisms leading lithosphere during periods of tectonic rejuvena- to large-scale intraplate failure (e.g. Vilotte et al. tion. This generally involves processes such as the 1982; England & Houseman 1985; Kuzsnir formation of new structural fabrics, the over- & Park 1987; England & Jackson 1989; Platt & printing of metamorphic assemblages and the England 1994; Tommasi et al. 1995; Ziegler et al. generation and emplacement of magmas. Such 1995, 1998; Avouac & Burov 1996; Neil & behaviour can occur repeatedly throughout the Houseman 1997; Sandiford & Hand 1998; geological record because the quartzofeldspathic Hand & Sandiford 1999; Pysklywec et al. 2000). continental crust cannot be subducted due to A number of factors are likely to control the its relative buoyancy and weakness compared locus of tectonic activity, but there appear to be with its oceanic counterpart and the underlying two first order controls: (1) temporal and spatial lithospheric mantle. Thus, the character of the variations in the thermal state of the lithosphere continents is significantly influenced by the way (e.g. Sonder & England 1986; England 1987; in which the existing lithosphere responds to Neil & Houseman 1997); and (2) the presence new tectonothermal events that follow geologi- of pre-existing mechanical defects such as faults, cally significant cessations of activity for mil- shear zones or major compositional boundaries lions to hundreds of millions of years (Sutton & (e.g. Ziegler et al. 1995; Butler et al. 1997; Holds- Watson 1986). worth et al. 1997). Existing continental lithosphere may be mod- The rejuvenation of pre-existing crust and ified during its incorporation into new collisional lithosphere occurs largely via two related pro- systems, for example the involvement of the cesses. Reactivation is normally considered to Hercynian 'basement' in the Alpine collision. involve the rejuvenation of discrete structures However, the most dramatic manifestations of (e.g. Holdsworth et al. 1997), whilst reworking continental tectonic rejuvenation occur during involves the repeated focusing of metamorphism, intraplate orogeny, where a coherent pre-existing deformation and magmatism into the same lithospheric volume undergoes large-scale failure. crustal- or lithospheric-scale volume. These are Notable modern examples of intraplate orogeny considered to be useful end-member definitions are the Cenozoic Tien Shan and the Mongolian that describe the way in which continental litho- Alti in north Asia, which are forming in response sphere is modified. However, there is some ambi- to the Himalayan collision (e.g. Hendrix et al. guity firstly because reworking and reactivation 1992; Dickson Cunningham et al. 1996), and may represent broadly the same process operat- also the Atlas Mountains of Morocco, which ing at different scales and/or depths (see below), From: MILLER, J. A., HOLDSWORTH,R. E., BUICK,I. S. & HAND, M. (eds) ContinentalReactivation and Reworking. Geological Society, London, Special Publications, 184, 1-12. 1-86239-080-0/01/$15.00 © The Geological Society of London 2001. Downloaded from http://sp.lyellcollection.org/ by guest on September 30, 2021 2 R. E. HOLDSWORTH ET AL. and secondly because there is some confusion the distribution and intensity of crustal heat regarding the use of these terms in the geological production is strongly controlled by the pattern literature. Several studies refer to the 'reactiva- and extent of denudation associated with orogen- tion' of mobile belts, but as these represent esis, which in turn may influence subsequent crustal volumes rather than discrete structures, spatial and temporal variations in the litho- they would be more accurately described as spheric thermal regime (e.g. Sandiford & Hand being reworked. Other terms used in the litera- 1998). Thus the consequences of one stage of con- ture include 'renewal' and 'remobilisation', both tinental evolution can potentially shape the pat- of which refer to the superposition of younger tern, style and distribution of subsequent events. geological events onto older geological systems. Since continental reworking involves diffuse The published geological literature suggests lithospheric-scale deformation, the behaviour of that continental reworking and reactivation can the mantle lithosphere is likely to be central in be expressed in a large number of ways, and determining the style and duration of the tec- are likely to arise for a range of complex rea- tonic activity. The potential role of the mantle sons. Thus, attempts to characterize the style lithosphere in continental reworking is reviewed and distribution of reactivation and reworking and investigated by Houseman & Molnar in the in different continental settings should provide first paper of this volume. They suggest that key datasets with which to evaluate the nature, localized lithospheric thickening associated with distribution and dynamic controls of tectonic plate convergence can potentially affect the gravi- rejuvenation in the continental crust and litho- tational stability of a layered system in which sphere. In editing this book, two end-member a dense non-Newtonian lithosphere overlies a approaches are recognized. The first takes a less dense fluid asthenosphere. Where instabil- lithospheric-scale perspective and essentially con- ity arises, a relatively strong crust will lead to siders how the continental lithosphere may localized downwelling beneath the centre of the respond to evolving first order variables, such convergent zone, whilst a relatively weak crust as the density structure of the mantle and changes will lead to downwelling on the margins of the in convergence style and rate. The second convergent zone as the buoyant crustal layer approach is case-study oriented. The two resists thickening. The initial instability may approaches are complementary and wherever then trigger rapid extension of the lithospheric possible should be integrated. Case studies pro- mantle beneath the orogen which is driven by vide crucial temporal and spatial information asymmetric cold downwellings that move away concerning the thermo-mechanical evolution of from the centre of the convergent zone. Using the crust and mantle in a variety of settings. examples of modern orogens from Southern Geochronological studies are particularly impor- California, South Island New Zealand, the tant in this context as they can provide clear Mediterranean, and Central Asia, Houseman & evidence for reworking or reactivation, and an Molnar show that there is strong evidence in each absolute timescale of events. The conceptual case that the mantle lithosphere has devel- approach can and should provide the stimulus oped some form of instability that has led to at for the collection of targeted datasets that seek least part of it being replaced by hot astheno- to evaluate the interactions of potential beha- sphere. Interestingly, teleseismic tomographic viours and causative mechanisms. images from these areas suggest that mantle lithosphere has been locally renewed following Continental reworking gravitational instability triggered by orogenic convergence. In this context, the time scales of Continental reworking encompasses structural, reworking should be linked to the rates at which metamorphic and magmatic processes that mod- downwelling of gravitationally unstable litho- ify existing continental lithosphere at an orogenic sphere occurs. Although these time scales are scale. An important difference between rework- not well known, Pysklywec et al. (2000) have ing and reactivation is that older structures do not suggested that crustal deformation driven by necessarily control the style, orientation and scale this process should have a duration of c. 25- of later structures. This is evidenced by orogenic 40 Ma. Rey also considers the interplay between belts such as the Cambrian Prydz-Leeuwin oro- the convective and lithospheric mantle in terms genic system which linked Africa, Antarctica and of its potential role in controlling the style of Australia,

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